This invention relates to a system for controlling the amount of electrical load connected to an electrical supply bus in an aircraft. More particularly, the invention relates to a system by which certain predetermined electrical loads are automatically disconnected from the aircraft supply bus, in relation to the maximum safe power supplying capacity of the electrical generators supplying the supply bus. The invention is particularly useful in and applicable to an aircraft of the type having a plurality of electrical power generators separately supplying electrical power to the supply bus and an automatic switching arrangement for automatically connecting one or more electrical load devices to the supply bus upon the movement of a wing flap or slat or other control surface of the aircraft from a retracted position to an extended position.
In the type of aircraft to which this invention is directly applicable, which comprises a significant portion of the commercial passenger-carrying aircraft throughout the world, two electrical fans known as pack fans are automatically connected to the supply bus and are energized when the wing flaps are extended. The pack fans aid in cooling the two passenger compartment air-conditioning, heating and pressurizing units which are known as pack units. The two pack units are pneumatically driven by heated and compressed air supplied from the compressor section of the three jet propulsion engines of the aircraft. When the pack units operate considerable heat is generated. The heat must be removed to prevent destruction of the pack unit and assure continued proper operation.
To remove the heat generated by the pack units air is ducted from the outside of the aircraft over the pack units. Under normal high-speed, high-altitude operating conditions the pack units are cooled by air forced through the duct and over the pack units by movement of the aircraft through the air. At aircraft speeds above 200 knots sufficient natural air flow is available to adequately cool the pack units. However, when the speed falls below 200 knots it is desirable to supplement the natural air flow by activating the pack fans which supplement the air flow over the pack units. The normal conditions under which the speed of the aircraft falls below 200 knots are low-altitude, slow-speed operating conditions, such as take-offs and landings and when the aircraft is parked. Under relatively high-speed normal flying conditions the pack fans are not needed and are deactivated.
In aircraft of the type described, an automatic switching system automatically energizes the pack fans when the wing flaps are extended. The pack fans are energized by connecting them to the aircraft electrical supply bus. The position of the wing flaps was selected as the determining factor for automatically energizing the pack fans because the wing flaps must normally be extended to sustain relatively slow flight speed, and slow flight speed is the condition where the pack fans are needed. By automatically energizing the pack fans in relation to the wing flap position, the pilots and flight engineers are relieved of responsibility of manually operating the pack fans during take-offs and landings, the most critical time of flight when many other matters must be controlled and supervised. This automatic system of energizing the pack fans in relation to the wing flap position, has, however, created serious problems and safety hazards, some of which have actually resulted in loss of life and aircraft.
The pack fans are the major electrical load in the aircraft. The pack fan electrical load is significantly greater than that electrical load created by the aircraft lights, safety and navigation equipment and instruments, radios, motors and other operational devices. The electrical load in this type of aircraft is supplied by three electrical generators, each of which is separately driven by one of the three jet propulsion engines of the aircraft. Under normal operating conditions, three properly operating electrical generators will supply sufficient power to energize the whole aircraft electrical system including the two pack fans. Potential overload problems result, however, when less than all three generators are operating.
Currently existing and authorized operations procedures sanctioned by the aircraft manufacturer, most air carriers, and the governmental agency charged with overseeing operation of passenger aircraft in the United States, allow operation of this type of aircraft when two of the three electrical generators are functioning. These authorized operating procedures explicitly require, however, that one of the two pack units including its pack fan must be manually disconnected from the electrical supply bus and rendered inoperable when the flaps are extended to assure safe operating conditions. Operating both pack fans and pack units from only two generators usually overstresses the two operating generators and causes them to trip off from an overloaded or overstressed condition. Of course, as each of the generators drops out or trips off, the magnitude of the overload becomes even greater for each remaining generator. The remaining generators will quickly trip off unless the overload condition is reduced or eliminated. For example, if an additional one of the last two functioning generators becomes inoperative, the last functioning pack unit and pack fan must be completely disconnected from the system to avoid an overload. If the last remaining pack fan is not disconnected, the last generator will become overstressed and will trip off, thus terminating the supply of electrical energy from the three generators driven by the jet propulsion engines. So long as the pack fans are manually disconnected in relation to the number of functioning generators, safe operation of the aircraft is possible. However once all pack units have been disabled, the passenger compartment is without pressurization, heating or cooling, which itself is an emergency condition.
In-flight problems can occur very quickly and require very rapid human response in order to avoid disaster. Generators and their control circuits have been known to malfunction. Jet propulsion engine problems, such as fire, flame-out or foreign object ingestion, also result in loss of a generator because stopping operation of the engine also terminates operation of the generator driven by that engine. Safety devices will automatically disconnect any malfunctioning of nonfunctioning generator from the electrical bus. The flight engineer must respond quickly to any of these inflight problems and disconnect certain electrical loads by noticing the problem, perceiving that the number of functioning generators has been reduced, and manually disconnecting some of the nonessential electrical load in order to avoid an electrical overload. It is not uncommon that the flight engineer is required to perform these activities under the stress of emergency conditions not principally related to electrical problems, such as occur from engine problems. The flight engineer may thus be required to be performing other safety precaution measures at the same time as reducing the electrical load on the aircraft.
It is possible that all jet propulsion engine driven generators may be lost or might trip off under certain circumstances. The type of aircraft described does have a battery system designed to supply sufficient electrical power for the emergency systems of the aircraft under conditions of loss of all three generators. The flight engineer must, however, manually condition the electrical system so that the battery system will become operative under these conditions. Failure to properly condition the electrical system or a failure of the battery system itself, can result in a total loss of electrical power within the aircraft.
The potentially-dangerous electrical overloading problem in this type of aircraft is present because the automatic switching arrangement automatically connects both pack fans to the electrical supply, but requires human intervention to disconnect additional load when an overload problem occurs. Additionally, the electrical overload problems are most apt to occur at times when flight of the aircraft is in its most critical stages--take-offs and landings. Electrical problems can adversely affect the control over the flight of the aircraft, and loss of control, even momentarily during take-off and landing, can be and has proved to be disastrous.
An example of an actually documented situation illustrates the significance of the problem to which the present invention relates. A major United States air carrier was operating an aircraft of the type of which this invention relates with only two of its three generators functioning, as allowed by authorized operating procedures. Because only two of the three generators were functioning, the flight engineer had manually disconnected one of the two pack fans and pack units prior to take-off of the aircraft. The flight began at night under conditions of low cloud cover and rain, thus requiring instrument take-off and flight. The flight originated at a coastal city airport, and the take-off path followed a course over the ocean. Within two minutes after leaving the ground and before an altitude of 3,000 feet had been reached, a warning signal indicated a fire in one of the three engines. Of course, the engine from which the fire warning signal originated was immediately shut down. The particular engine which was shut down happened to be one of the engines driving one of the two operational electrical generators. Consequently, only one electrical generator remained to supply all of the electrical needs of the aircraft. Shortly after shut down of the engine, all of the electrical power was lost when the last functioning generator tripped off. The navigational instruments and lights became inoperative because of the failure of the electrical power system. The pilot lost spacial orientation in the darkness and cloud cover, and the attitude of the aircraft changed from the upright climb to an inverted, high-speed, steep dive toward the ocean. The aircraft disintegrated and all occupants perished when the aircraft crashed into the ocean. Evidence recovered from the crash site indicated that the wing flaps were in the extended position when the crash occurred. Although some evidence suggested that the flight engineer had attempted to reduce the electrical load after the fire warning, the last remaining generator tripped off within seconds after the previously functioning second generator was cut off when its driving engine was shut down. It could be that the attitude change of the aircraft coupled with the loss of interior lights and the exigencies of the fire warning may have resulted in the failure to manually disconnect nonessential electrical loads, such as the one operating pack fan.
The present invention is addressed to these and other problems, potential problems and potential safety hazards, which heretofore may have not been appreciated or perceived in the light described here, but which nonetheless are present in aircraft of the type in which the pack fans are automatically connected to the electrical system in relation to the extension of the wing flaps. Those skilled in the art may more fully appreciate the significance of the present invention in light of a more complete understanding of the nature and limitations of the type of aircraft to which this invention relates.